This proposal will test the hypothesis that up-regulation of the HEF1 gene in breast tumors is an important contributing factor for cancer metastasis. This is based on the ability of elevated HEF1 to drive proliferation and invasion. For invasion, cells become increasingly motile and acquire the ability to invade surrounding tissue which enables metastasis. A series of studies within the past year have nominated the HEF1 protein as an essential switch for pro-metastatic behavior in tumors. These studies have identified HEF1 as a component of a small signature of genes upregulated in metastasizing breast adenocarcinomas, shown HEF1 is important for glioblastoma invasiveness, and determined that upregulation of HEF1 occurred in more than 30% of metastatic melanomas. In this proposal, we will address two basic questions: 1) How does absence or overexpression of HEF1 condition AurA activation, and 2) How does HEF1 regulate invasion in breast cancer cells? In specific experiments to answer these questions, Aim 1 will use mechanistic, cell-based assays to investigate how HEF1 regulates AurA degradation. We will test the hypothesis that HEF1 protects AurA from ubiquitin-dependent proteosome degradation and determine the impact of the HEF1-AurA complex on efficacy of clinically relevant AurA inhibitors. Aim 2 will define the molecular mechanisms underlying HEF1's invasion promoting abilities. We will test the hypothesis that HEF1-activated AurA activates histone deacetylase HDAC6 and promotes deacetylation of invadopodia seeding component- cortactin. In Aim 3 we will test the hypothesis that elevated HEF1 expression contributes significantly to breast cancer metastasis using orthotopic mouse xenograft models with inducible shRNAs against HEF1 and modern multimodal non-invasive fluorescent/bioluminescent imaging technology to establish molecular mechanisms of HEF1-driven tumor growth and metastasis in human cancer. The ultimate goal of these experiments is to improve the diagnosis and treatment of cancer in human patients.